Modern industrial printing machines, such as those used to print drink cans, often operate at high speeds in the range of 1,800 to 2,000 cans per minute. Drink can customers desire clear, sharp images and a high degree of consistency in image quality to support demanding advertising standards. The desire for crisp images often requires sharp, laser engraved images on the printing plates that create the images transferred onto the cans. There are a number of pliable and steel rollers in the ink train that transfer ink from a fountain and apply the cans. Certain pliable rollers known as the form rollers, which apply the ink directly to the engraved printing plates, are subject to the most intense wear and tear from coming into direct contact with the sharp edges of the print plates. The form rollers must also maintain a precise pressure to apply the desired thickness of ink to the printing plates while rotating at sufficient speed to support the high speed printing process.
The form rollers therefore tend to breakdown quickly requiring frequent replacement. In a conventional printing machine, the form rollers often require replacement every six to eight hours making them the most frequently replaced component in the assembly line dictating the machine duty cycle. The ink train may also include a number of other pliable rollers typically referred to as the ductor and transfer rollers. Although not subject to the same level of wear and tear as the form rollers, these other pliable print rollers also require regular maintenance and replacement on a regular basis. Replacing the print rollers requires shutting down the assembly line, which reduces the overall output and efficiency of the printing machine.
Holding other variables constant, harder pliable rollers generally last longer. But increasing the hardness of the pliable rollers presents a trade off in that harder rollers require higher pressure to apply the desired ink coating thickness, which increase energy usage, heat generation, and wear and tear on the rollers. The form rollers, in particular, are required to apply an ink coating having a desired thickness to the printing plates carried on the plate cylinder, which is typically measured as the width of the line laid down by a non-rotating form roller at a given pressure. For example, the line thickness standard may be 3/16 inch [0.476 cm], and a form roller with a 55 shore durometer hardness may require six lbs [2.7 kg] per inch [2.54 cm] of roller length (e.g., 30 lbs [13.6 kg] pressing a five inch [12.7 cm] roller against the plate cylinder) to apply a standard line width of 3/16 inch [0.476 cm]. On the other hand, a harder roller with a 65 shore durometer hardness may require eight lbs [3.6 kg] per inch [2.54 cm] of roller length (e.g., 40 lbs [18.1 kg] pressing a five inch [12.7 cm] roller against the plate cylinder) to apply the same standard line width of 3/16 inch [0.476 cm].
While harder rollers are generally expected to be more durable, they also require higher roller pressure to print the desired line width which, in turn, increases heat generation within the roller and wear and tear on the roller. As a result, harder rollers are not necessarily more durable in practice. Moreover, using harder rollers increases machine drag and energy consumption. Harder rollers may also experience lower surface resiliency causing them to perform poorly at higher pressures and machine speeds. As a result, the tradeoff balance conventionally adopted by the industry utilizes relatively firm form rollers (e.g., 60-65 shore durometer) that print crisp images at reasonably obtainable roller pressures (e.g., 35-40 lbs [15.9 to 18.1 kg] on a five inch [12.7 cm] roller) but require relatively frequent replacement (e.g., every six to eight hours). Having to shut down the printing machine to change out the form rollers every six to eight hours limits the continuous run duty cycle and is considered to be a major limitation in high speed drink can printing.
As a result, there is significant need for improved pliable print rollers with improved durability while still meeting the print quality, roll pressures, and line speeds demanded by modern industrial printing processes.